The production of reinforced thermoplastic laces by using powdered thermoplastic polymers to impregnate continuous rovings of fibrous material, particularly glass fibre, is disclosed in British patent specifications Nos. 1 259 085 and 1 334 702 and U.S. Pat. No. 3 586 560 and 3 673 027. These specifications describe processes in which powdered thermoplastic is applied by various methods to the filaments making up a continuous roving and which are subsequently heated to melt the polymer. The impregnated rovings are generally pulled through forming surfaces such as dies to consolidate and remove excess polymer from the impregnated roving. The impregnated rovings produced by these processes, also known as "laces", can be cut into granule form for use in injection moulding machines. Although advantages are obtained by the use of powder impregnation as compared with the more conventional polymer melt impregnation processes it is normally found that the various processes involved in producing the finished lace result in higher tension being generated in the lace as it is pulled through the equipment used in these processes than when using the melt impregnation processes. Under the conditions operating for the major part of the powder-impregnation process, the tension in the lace will remain reasonably constant. Conditions may occasionally arise, however, for example because an excessive amount of polymer has been picked up by the roving, when the tension in the lace being hauled off can increase to such an extent that the lace breaks. Such breakage can be extremely inconvenient resulting in delays whilst the blockage is cleared and the broken lace re-threaded through the system of impregnation chamber, heating chamber, consolidation die etc to the haul-off equipment for the lace. An improved method of producing reinforced laces which reduces lace breakage arising from an increase in lace tension for any reason has now been developed.